In photographic silver halide emulsion layers gelatin remains the principal binder employed in the art.
The demand for rapid processing, dimensional stability, and image sharpness has led to a steady decrease in gelatin-AgX ratios to give thinner emulsion layers, e.g. 10 micron thick for colour reversal emulsions and 5 micron thick for rapid access medical X-ray films (ref. SPSE Handbook of Photographic Science and Engineering--Woodlief Thomas, Jr. Editor, SPSE--A Wiley Interscience Publication--John Wiley & Sons, New York, (1973), p. 514).
From the book "Photographic Chemistry" by Pierre Glafkides, Vol. I, (1958), p. 314-315 it is known that a silver halide emulsion containing little gelatin with respect to the silver halide will develop more rapidly, and that the gamma infinity will be reached more quickly.
A decrease in the amount of gelatin and the reduction of the coating thickness brings about a series of problems of which coating uneveness and coating streaks are the most important.
In coating a gelatin containing layer the viscosity of the coating liquid is of particular importance in the choice of the coating technique. Modern coating apparatus used for high speed coating of gelatin containing aqueous coating liquids on web type film or paper supports are the slide hopper coater and the curtain coater. Examples of curtain coaters are described in U.S. Pat. Nos. 3,632,374, 3,867,901 and examples of slide hopper coaters are described in U.S. Pat. Nos. 2,761,791, 4,113,903 and published EP-A 0 382 058.
When carrying out a method operating with high coating speed it is of utmost importance that after coating the gelatin containing layer on cooling sets or solidifies as rapidly as possible and its consistency reaches rapidly a degree allowing the drying, i.e. the removal of water from the coated layer, with a vigorous dry air current without distorting the already solidified layer containing gelatin in gel form. Only by combining rapid coating speed with short solidification and drying stages can the production speed of recording materials based on (a) gelatin containing coating(s) be increased substantially.
It is known that silver halide emulsions with very low viscosities are more prone to instability in the coating bead before contact with the web, which causes coating defects, and undesired interlayer mixing in multilayer coating. Various thickening agents have been proposed to facilitate the coating of dilute photographic emulsions as described e.g. in U.S. Pat. No. 3,767,410. Many of these agents are not entirely satisfactory and they exhibit problems such as the production of haze, brittleness, and for anionic macromolecular polyelectrolytes restricted use in a particular pH environment because of undesired coagulation of the gelatin in their presence.
As a practical matter, e.g. with the objective of manufacturing low gelatin-content photograph gelatin silver halide emulsion layer materials, it is therefore preferred to eliminate any thickening agents and still to obtain a desired viscosity in the gelatin containing coating liquids correlated with a proper shear rate for fault-free coating.
The use of thickening agents, more particularly anionic macromolecular polyelectrolytes, with the aim to obtain gelatin solutions having strong shear rate dependency is in many cases unfavorable for rapid solidification of the coated layer.
The coating of low gelatin-content silver halide emulsion layers is particularly important when ultra rapid processing is sought by which is meant processing within 20 to 60 seconds in total time during which the development, fixing, water rinsing and drying of a given point on the photographic material is completed.
Ultra-rapid processing proceeds normally at elevated temperature, e.g. in the temperature range of 30.degree. to 45.degree. C. in automatic roller transport apparatus. Under such conditions low gelatin content silver halide emulsion layers have too low an abrasion resistance, may show roller marks and give rise to sludge formation.